1. Field of the Invention
The present invention relates to a fixing device for an image forming apparatus, such as a printer, a facsimile machine or a copier, and relates to an image forming apparatus including a fixing device.
2. Description of the Related Art
Fixing devices are used in various image forming apparatuses, such as copiers, printers, facsimile machines or multi-function peripherals. Among them, a fixing device including a thin-walled fixing belt which is implemented by a metallic base plate and an elastic layer is known. The thin-walled fixing belt provides a reduced thermal capacity and the energy required for heating the fixing belt may be substantially reduced. Further, with the thin-walled fixing belt, a warm-up time and a first print time of the fixing device may be shortened.
For example, Japanese Laid-Open Patent Publication No. 2007-233011 discloses a fixing device of this type. FIG. 16 shows a composition of a fixing device 120 according to the related art as disclosed in Japanese Laid-Open Patent Publication No. 2007-233011. As shown in FIG. 16, in this fixing device 120, a nip formation member 124 is disposed on an inner peripheral surface of an endless fixing belt 121, and a pressure roller 122 is made to contact the nip formation member 124 via the fixing belt 121. A nip portion N is formed between the fixing belt 121 and the pressure roller 122. When the pressure roller 122 is rotated by a drive unit which is not illustrated, the fixing belt 121 follows the rotation of the pressure roller 122 and is rotated.
The nip formation member 124 is supported on an inner peripheral surface of the fixing belt 121 by a support member 125 and both ends of the support member 125 in a longitudinal direction thereof are fixed to side plates of a body of an image forming apparatus. The support member 125 includes a parallel portion 125a and a rise portion 125b. The parallel portion 125a is parallel to a transport direction of a sheet P at the nip portion N and supports the nip formation member 124. The rise portion 125b vertically projects upward from the parallel portion 125a in a direction to go away from the nip portion N 124. The support member 125 serves to increase the rigidity of the nip formation member 124 against the pressing force of the pressure roller 122.
A heat source 123 which is a heating unit to heat the fixing belt 121 in a region fully covering a width of a sheet by radiant heat is disposed on an inner peripheral side of the fixing belt 121. The fixing belt 121 except the portion where the nip formation member 124 is disposed may be directly heated by the radiant heat from the heat source 123, and the efficiency of heat transfer from the heat source 123 to the fixing belt 121 may be substantially increased. Accordingly, the power consumption may be reduced and the first print time from the heating standby state may be shortened further.
In the above-described fixing device, the fixing belt is directly heated, and if the diameter of the fixing belt is reduced to a diameter of approximately 30 mm, the heat consumption region where heat dissipates from the fixing belt may be reduced and thermal efficiency may be increased.
Unlike a time of stopping the rotation of the fixing belt or during a stable rotation state, when the pressure roller is initially actuated to rotate the fixing belt at a start of operation of the fixing device, a dynamic load from the pressure roller is applied to the fixing belt in a direction from the end of the nip portion toward the center of rotation of the fixing belt. Hence, an oblique-direction force due to the dynamic load is exerted on the nip formation member in a direction oblique to the sheet transport direction in the nip portion so that bending or torsion of the nip formation member may take place. If the support member has sufficient rigidity, the occurrence of bending or torsion of the nip formation member at this time may be prevented.
However, if a small-sized fixing belt is used, the support member disposed on the inner peripheral surface of the fixing belt is usually small-sized. The rigidity of the parallel portion or the rise portion of the support member is reduced (or falls) depending on the composition of the support member in such a case, and it is difficult to obtain sufficient rigidity of the support member.
At a start of the rotation of the fixing belt, the oblique-direction force due to the dynamic load is exerted on the parallel portion of the support member through the nip formation member. At this time, the rise portion may support the parallel portion against the oblique-direction force, and the occurrence of bending or torsion in the parallel portion may be prevented more positively than in a case where the support member has no rise portion.
However, the rise portion projects vertically upward from the parallel portion, and the oblique-direction force due to the dynamic load is applied to the rise portion via the parallel portion in a direction to bend the rise portion. Hence, if the rigidity of the rise portion falls as described above, bending or torsion of the rise portion may take place and the effects to prevent the occurrence of bending or torsion of the parallel portion by the rise portion may be reduced. As a result, the occurrence of bending or torsion of the nip formation member which is supported by the parallel portion may not be prevented. In such a case, the pressure pattern of the nip portion may vary, the nip width may vary, and poor fixing may arise.